OLETs enter organic emitter race

06 May 2010

Details of an organic light-emitting transistor that outperforms the equivalent OLED have been published.

Organic light-emitting transistors (OLETs) could usher in a new era of organic optoelectronics, say researchers from Italy and the US. By creating an architecture that prevents the traditional loss mechanisms found in OLEDs, the team says that the performance of its OLET exceeds that of the best OLED based on the same emitting layer (Nature Materials doi:10.1038/NMAT2751).

"Our tri-layer OLETs represent a viable route towards practical organic light-emitting devices with unprecedented performances for integrated photonic bio-sensing and high-resolution display technology with embedded electronics," Michele Muccini from the Institute for Nanostructured Materials CNR Bologna told optics.org. "OLET is a new light emission concept, providing planar light sources that can be easily integrated in substrates of different nature (silicon, glass, plastic, paper, etc) using standard microelectronic techniques."

An OLET is an optoelectronic device that has the structure of a thin-film transistor and the ability to emit light. Although its cousin the OLED is a more established and commercially available technology, exciton quenching and photon loss processes still limit the efficiency and brightness of today's OLEDs.

"The unprecedented control of quenching and loss processes offered by OLETs allows us to tackle these fundamental OLED issues," commented Muccini. "Our work has been motivated by the need to unravel the full potential of field-effect transistors as a photonic technology platform."

Muccini and his colleagues have developed a p-channel/emitter/n-channel tri-layer semiconducting heterostructure that they say dramatically improves OLET performance. The device is fabricated on glass and indium tin oxide and the central light-formation layer is a blend of Alq3 and DCM. According to Muccini, the major challenge was to fabricate a continuous multi-stack of different organic materials, with interfaces compatible with the functions of field-effect charge transport and light emission.

"Our OLET devices are more than 100 times more efficient than the equivalent OLED, more then two times more efficient than the optimized OLED with the same emitting layer and more than 10 times more efficient than any other reported OLETs," said Muccini. "We report OLET external quantum efficiencies of 5%. In addition, we have shown that the same organic emitting layer leads to more efficient device emission when it is incorporated in the OLET structure than in the OLED one."

The team is now looking to optimize and improve its OLETs even further. "A critical parameter to be addressed is the device operating voltage," commented Muccini. "We hope to achieve lower operating voltages by using high-capacitance gate insulators. We will also be improving light confinement, guiding and extraction. Device reliability and lifetime under operational conditions also need to be thoroughly addressed."